📖 Complete Directory of SMTP Error Codes & Email Bounce Meanings

Hard bounces, soft bounces, authentication failures, and provider-specific errors, decoded.

What Is This Directory?

There is nothing more frustrating than seeing your email campaign hit with a wave of “Message Undeliverable” notifications. Understanding SMTP error codes is the first step toward fixing your email deliverability and ensuring your messages actually land in the inbox.

Whether you are dealing with a hard bounce (5xx permanent failure) or a soft bounce (4xx temporary failure), this complete directory decodes the meanings behind the most common email bounce errors. From DNS and authentication failures like SPF and DMARC violations to recipient issues like mailbox over quota, we’ve categorised these errors into tiers to help you prioritise your troubleshooting.

How to Use This Guide

🔴Tier 1 (Critical): High-frequency errors that require immediate attention to protect your sender reputation.

🟡Tier 2 (Common): Routine delivery hurdles and technical “handshake” issues.

📌Status Indicators: Look for 5.x.x for permanent failures and 4.x.x for temporary issues that may resolve with a retry.

🔍Use the list below to identify your specific error code and learn exactly why your email didn’t reach its destination.

Tier 1: Very Common

🔴 550 5.1.1

Mailbox Does Not Exist

Message Too Large for Mailbox

🔴 552 5.3.4

Message Too Big for System

🔴 550 5.4.1

Recipient Address Rejected

🔴 550 5.4.4

No Route to Destination

🔴 550 5.7.1

Message Rejected by Policy

🔴 535 5.7.8

Authentication Credentials Invalid

🔴 550 5.7.13

User Account Disabled

🔴 550 5.7.14

Trust Relationship Required

🔴 550 5.7.23

SPF Validation Failed

🔴 550 5.7.26

DMARC Policy Violation

🔴 550 5.7.515

Outlook Rejects Unauthenticated High-Volume Sender

🟡 450 4.2.1

Rate Limit: Try Again Later

🟡 452 4.2.2

Mailbox Full (Temporary)

Message Expired in Queue

🟡 421 4.7.0

Temporary Policy Rejection

🟡 421 4.7.28

Blocked: High Complaint Rate

Tier 2: Common

🔴 550 5.1.0

Other Address Status

🔴 550 5.1.2

Bad Destination Domain

🔴 501 5.1.3

Invalid Address Syntax

Mailing List Expansion Problem

🔴 550 5.3.0

Other Mail System Status

🔴 421 5.3.2

System Not Accepting Messages

🔴 554 5.4.6

Routing Loop Detected

🔴 550 5.4.7

Delivery Time Expired

🔴 550 5.7.0

Message Rejected (Policy)

🔴 550 5.7.2

Mailing List Expansion Prohibited

🔴 534 5.7.9

Authentication Mechanism Too Weak

🔴 550 5.7.24

SPF Validation Error (PermError)

🔴 550 5.7.25

Reverse DNS (PTR) Validation Failed

🟡 421 4.3.2

System Not Accepting Mail

🟡 451 4.7.1

Temporary Auth / Policy Hold

🟡 421 4.7.29

Blocked: Local Policy (New Sender)

🟡 421

Service Temporarily Unavailable

🟡 450

Requested Action Not Taken (Temporary)

🟡 451

Local Processing Error (Temporary)

🟡 452

Insufficient Storage (Temporary)

🔴 500

SMTP Syntax Error

🔴 501

Bad Parameter or Argument

🔴 550

Generic Delivery Failure

🔴 554

Transaction Failed

Provider-Specific Error Code Directories

Each major provider uses proprietary codes and identifiers inside standard SMTP responses. Use these dedicated directories to decode the exact bracketed codes you see in your mail logs.

Google / Gmail

gsmtp · gcdp identifiers

5 critical errors →

Microsoft 365

RESOLVER · STOREDRV codes

5 critical errors →

Yahoo / AOL

[TSS] · [TS] deferral codes